It is known that such a slaving chain comprises electronic components which are liable, in failed mode, to generate a spurious signal which may cause the slaved airfoil to oscillate. A phenomenon of this type is called an “oscillatory fault”. Another possible cause of the oscillation is the malfunction or breakage of a mechanical part of the actuator.
It is known moreover that, when an oscillatory fault such as this exhibits a frequency which lies inside the bandwidth of the actuator, its effect is:                to generate significant loads on the structure of the aircraft, thereby making it necessary to reinforce this structure, if a dedicated device does not enable this fault to be detected;        to generate excessive loads in the event of excitation of one of the natural modes of vibration of the aircraft (phenomenon of resonance, aeroelastic coupling), and this may, in the extreme case, result in damage to the structure of the aircraft;        to accelerate the fatigue of the actuator or actuators used; and        to reduce the comfort of the aircraft's passengers.        
Complete coverage of oscillatory faults such as these would require overly expensive structural reinforcements of the aircraft. In practice, the aircraft is designed to absorb oscillatory faults of a certain amplitude, as a function of frequency. So, monitoring must be put in place to guarantee that the vibrations of the aircraft remain inside a predetermined amplitude/frequency envelope.
Nevertheless, the standard solutions for carrying out such monitoring are highly dependent on:                the hardware used;        the type of law for piloting the aircraft (dependent on the latter's flexibility or inflexibility);        the computer acquisition and generation system; and        the fault modes of said computer.        
Consequently, to a particular family of aircraft there always corresponds a particular standard solution, which does not exhibit any guarantee of being applicable to another, existing or future, family of aircraft.
Moreover, the standard monitoring solutions generally exhibit restricted coverage, usually only undertaking detection of the oscillations generated by a particular component of the slaving chain.
The object of patent application FR-05 12000 is to at least partially remedy these drawbacks. It relates to a method, which is robust and which is applicable to any type of aircraft with electric flight controls, for detecting at least one oscillatory fault in at least one positional slaving chain for at least one airfoil of the aircraft, in particular a transport airplane, said method making it possible to detect an oscillatory fault of a minimum amplitude in a number of limited periods, doing so whatever the frequency of this oscillatory fault.
According to this patent application FR-05 12000, the following series of successive steps is carried out in an automatic and repetitive manner:    a) a theoretical position corresponding to a reference position of said airfoil in the absence of a fault is estimated with the aid of the airfoil control command which feeds a model of said actuator;    b) the difference between said theoretical position estimated in step a) and the effective position measured by said sensor is calculated so as to form a residual value; and    c) this residual value is compared with at least one predetermined threshold value, a count is carried out of all the successive and alternating overshoots of said predetermined threshold value by said residual value, and, as soon as the number resulting from said count becomes greater than a predetermined number, an oscillatory fault is detected which represents a periodic signal of sinusoidal type, whose frequency, amplitude and phase follow a uniform law (that is to say do not exhibit any favored values).
This patent application FR-05 12000 therefore makes provision to compare the actual operation of the monitored slaving chain (which is illustrated by the measured effective position), with an expected ideal fault-free operation (which is illustrated by said theoretical position), thereby making it possible to highlight any oscillatory fault when it arises. This comparison is performed by calculating a residual value. Consequently, by virtue of the method of detection of this patent application FR-05 12000, it is possible to detect, in the monitored slaving chain, any oscillatory fault of a given minimum amplitude in a given number of periods.
The model used in the aforesaid step a) comprises a plurality of parameters. Patent application FR-05 12000 provides for the use of constant values, fixed at a mean value, for these parameters. Now, in reality, some of the possible parameters of this model are indeed constants, but others evolve as a function of time. In particular:                a possible parameter which represents the difference between the differential supply pressure across the terminals of the actuator and the pressure at which valves for isolating the actuator open is, for example, dependent on the temperature of the hydraulic fluid and the number of consumers (actuators) on the hydraulic circuit;        a possible parameter illustrating the set of aerodynamic loadings which are exerted on the airfoil depends on a large number of variables, for example the dynamic pressure (therefore the speed of the airplane), the Mach number, the configuration of slats and flaps, the local incidence of the airfoil, etc.; and        a possible parameter illustrating the loading caused by the adjacent actuator in damped mode, in the case of two actuators per airfoil, depends mainly on the temperature of the hydraulic fluid.        
Consequently, several parameters of the model used in the aforesaid detection method depend on other quantities which vary as a function of time. So, the fact of fixing these parameters at constant values degrades the quality of said model. Now, the quality of this model is very significant, since it conditions the performance of the detection method. Specifically, the more one seeks to detect low fault levels, the higher the quality of the model must be. The use of constant parameters therefore limits the performance of the detection method disclosed by patent application FR-05 12000. So, for an existing airplane, if one were induced to decrease the fault level to be detected, this standard method could no longer be adapted to such a decrease. Furthermore, future airplane construction programmes will certainly seek to optimize the airplane globally so as to improve its performance. This could result in greater demands on the structural layout of the airplane (saving of mass) and therefore in more constraining detection levels and confirmation times. In such a situation, the aforesaid detection method would no longer be sufficiently efficacious.